#!/usr/bin/env python 
s = raw_input('Enter a score: ')
score = int(s)

if score >= 90 and score <= 100:
	print 'A'
elif score >= 80 and score <= 89:
	print 'B'
elif score >= 70 and score <= 97:
	print 'C'
elif score >= 60 and score <= 69:
	print 'D'
else:
	print 'F'
	
